Metoprolol, having the formula ##STR9## is a wellknown pharmaceutical compound having a strongly selective blocking effect on the cardiac .beta..sub.1 -receptors in animals and man, and being used inter alia in the treatment of hypertension, cardiac arrhythmias, angina pectoris and migraine. It is a well established fact that in metoprolol, like in other .beta.-blockers the .beta.-blocking effect resides only in the S-enantiomer, while possibly occurring side effects may reside in either or both enantiomers.
S-metoprolol and a process for preparation thereof is described inter alia in EP-A2-0 179 031.